Essentially, it is known that a thermal magnetic circuit breaker is a manually or automatically operating electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit, for example. Its basic function is the detection of a fault condition and the interruption of current flow. Therefore, the thermal magnetic circuit breaker has for example at least one magnetic trip device in order to prevent the electrical circuit or an electrical device from damage by short circuit and a thermal trip device in order to prevent the electric circuit or an electrical device, like a load, from damage by overload. A short circuit is an abnormal connection between two nodes of the electric circuit intended to be at different voltages. Moreover, especially in reference to a molded-case circuit breaker, a short-circuit is an abnormal connection between two separate phases, which are intended to be isolated or insulated from each other. This results in an excessive electric current, named an overcurrent limited only by the Thévenin equivalent resistance of the rest of the network and potentially causes circuit damage, overheating, fire or explosion. An overload is a less extreme condition but a longer-term over-current condition as a short circuit.
The thermal magnetic circuit breaker or breaker, respectively, has different settings or adjustments, respectively, as to where does the client wants the breaker to trip thermally. These settings go for example from 0.8 ln to 1 ln, wherein 0.8 ln means 80% of the nominal current rated on the breaker and 1 ln means 100% of the nominal current rated on the breaker. Therefore, in a 100 Amp breaker, 80% will be 80 Amp.
Basing on a lower thermal adjustment, less electrical current goes through a conductive element like a conductor and results on a lower temperature on a bimetal element of the thermal trip device. Thus, the temperature profile of the thermal trip device of the thermal magnetic circuit breaker or thermal magnetic trip unit (TMTU) presents low temperature behaviour on the lower thermal adjustment side, which is for example 80% In and therefore 80% of the nominal current, as mentioned above. Since the movement of the bimetal element is a result of the temperature, such a low temperature is not enough in order to reach deflection and force of the bimetal element of the thermal trip device, which are necessary to unlatch the breaker mechanism. Essentially, the bimetal element needs a temperature of circa 150° C. in order to reach a sufficient deflection and release a breaker mechanism after an overload fault in the thermal magnetic circuit breaker.
Therefore, the deflection of the bimetal element is not enough for doing contact to the breaker mechanism, when a temperature is reached low like for example circa 80° C. Therefore, a lower electrical current inducts a less temperature and therefore, a less deflection and/or force of the bimetal element, during a high electrical current inducts a higher temperature and as a consequence, a higher deflection and/or force of the bimetal element.
It is known that a tripping device like a tripping slide of the breaker mechanism or a latch mechanism, respectively, unlatched by the deflected bimetal element has a ramp feature that allow different distances of the bimetal element depending of the available temperature besides there is a calibration screw that makes precision. A calibrations screw needs a detailed time-consuming calibration of a customer or end user or an operator during the calibration process and therefore a detailed expertise about the field of application and so on.